Undesirable airborne compounds, including sulfur compounds, ammonia, formaldehyde, urea, carbon monoxide, oxides of nitrogen, mercaptans, amines, and ethylene, occur in a number of environments, where most are primarily responsible for the presence of disagreeable odors, or irritating or toxic gases. Such environments include petroleum treatment and storage areas, sewage treatment facilities, hospitals, morgues, anatomy laboratories, animal rooms, and pulp and paper production sites, among others. These undesirable compounds may be bacterial breakdown products of higher organic compounds, or byproducts of industrial processes.
Hydrogen sulfide ("H.sub.2 S"), a colorless, toxic gas with a characteristic odor of rotten eggs, is produced in coal pits, gas wells, sulfur springs, and from decaying organic matter containing sulfur. Controlling emissions of this gas, particularly from municipal sewage treatment plants, has long been considered desirable. More recently, protecting electronic apparatus from the corrosive fumes of these compounds has become increasingly important. Further, H.sub.2 S is flammable.
Ammonia ("NH.sub.3 "), also a colorless gas, possesses a distinctive, pungent odor and is a corrosive, alkaline gas. The gas is produced in animal rooms and nurseries and its control also has long been considered desirable.
Chlorine ("Cl.sub.2 ") is a greenish-yellow gas with a suffocating odor. The compound is used for bleaching fabrics, purifying water, treating iron, and other uses. Control of this powerful irritant is most desirable for the well-being of those who work with it or are otherwise exposed to it. At lower levels, in combination with moisture, chlorine has a corrosive effect on electronic circuitry, stainless steel and the like.
Formaldehyde ("HCHO") is a colorless gas with a pungent suffocating odor. It is present in morgues and anatomy laboratories, and because it is intensely irritating to mucous membranes, its control is desirable.
Urea ("CH.sub.4 N.sub.2 O") is present in toilet exhaust and is used extensively in the paper industry to soften cellulose. Its odor makes control of this compound desirable.
Carbon monoxide ("CO"), an odorless, colorless, toxic gas, is present in compressed breathing air. Oxygenation requirements for certain atmospheres, including those inhabited by humans, mandate its control.
Oxides of nitrogen, including nitrogen dioxide ("NO.sub.2 "), nitric oxide ("NO"), and nitrous oxide ("N.sub.2 O"), are compounds with differing characteristics and levels of danger to humans, with nitrous oxide being the least irritating oxide. Nitrogen dioxide, however, is a deadly poison. Control of pollution resulting from any of these oxides is desirable or necessary, depending on the oxide.
Mercaptans and amines, including methyl mercaptan ("CH.sub.3 SH"), butyl mercaptan ("C.sub.4 H.sub.9 SH") and methyl amine ("CH.sub.5 N"), are undesirable gases present in sewerage odor. The control of these gases is desired for odor control.
Ethylene ("C.sub.2 H.sub.4 ") is a colorless, flammable gas that is a simple asphyxiant which accelerates the maturation or decomposition of fruits, vegetables, and flowers. Control of this compound prolongs the marketable life of such items.
Attempts have been made to provide solid filtration media for removing the undesirable compounds listed above from fluid streams. Desired features of such media are a high total capacity for the removal of the targeted compound, a high efficiency in removing the compound from an air stream contacting the media, and a high ignition temperature (non-flammability).
The following approximate formulation is an example of a solid filtration media produced by the current state of the art: 69% activated alumina, 10% water, 4.5% potassium permanganate, and 17% sodium bicarbonate. The above solid filtration media is widely known in the art to have approximately a 9% capacity for the uptake of hydrogen sulfide gas in a gas stream.
One specific example of a solid filtration media for the removal of undesirable compounds from gas streams is described in U.S. Pat. No. 4,235,750. The '750 patent discloses an apparatus and method for adsorbing ethylene and other gaseous contaminants, wherein the apparatus is a three-part container comprising permanganate impregnated alumina in one compartment, activated carbon in the second compartment, and a mixture of molecular sieves and activated silica gel in the third compartment. The '750 patent discloses that the concentration of the aqueous potassium permanganate solution used to impregnate the alumina should be limited to one pound of permanganate dissolved in one gallon of water since if more is dissolved, the pores of the alumina will be clogged, therefore reducing its oxidizing capacity. Ideally, the potassium permanganate and water solution is applied to the substrate so that the dried, finished product contains about 4 to 5%, preferably 4.5% of potassium permanganate by weight of the finished product. Also, the '750 patent teaches that water is simply a diluent which will eventually be evaporated, and therefore, is not meaningful except as a vehicle for application of the potassium permanganate to the basic alumina substrate. The product is deemed finished after a substantial portion of the free water has been evaporated. Preferably, the alumina is dried to remove 99% of the free water after the aqueous potassium permanganate solution has been applied to the alumina.
Although the '750 patent discloses a potassium permanganate impregnated alumina for the removal of undesirable compounds from fluid streams, the capacity of the impregnated alumina is limited. The efficiency of the permanganate impregnated alumina of the '750 patent is limited as its optimal concentration of permanganate is 4.5%, and higher concentrations of permanganate results in the clogging of the pores of the substrate and therefore its oxidizing capacity being reduced. Accordingly, this filtration media would be limited to approximately a 9% capacity for the uptake of hydrogen sulfide gas in a gas stream. Therefore, this filtration media could not be efficiently used in small filter beds as larger quantities of the impregnated alumina must be used to compensate for its limited capacity. Further, the use of the impregnated alumina of the '750 patent would be more costly as the media would have to be replaced more frequently, thereby incurring the cost of more frequently purchasing the media and also incurring the cost of the additional labor required for its more frequent replacement. Finally, the permanganate impregnated alumina of the '750 patent is limited in that the failures in the adsorption of contaminants in fluid streams which occur at the end of the useful life of the media would be more frequent due to the limited capacity of the media. Therefore, the media of the '750 patent could not practically be utilized in systems where the air quality is critical.
Another example of a solid oxidizing system in pellet to form consisting of activated alumina ("Al.sub.2 O.sub.3 ") impregnated with potassium permanganate ("KMnO.sub.4 ") is described in U.S. Pat. No. 3,049,399. The pellets disclosed in the '399 patent provide air purification and odor control by both adsorbing and absorbing odors, and then destroy the collected odors by the potassium permanganate's controlled oxidizing action. Apparently, the permanganate destroys odors by the following oxidation reactions: EQU MnO.sub.4.sup.- +8H.sup.+ +5e.sup.- .fwdarw.Mn.sup.+2 +4H.sub.2 O (acid) EQU 3MnO.sub.4 +2H.sub.2 O.fwdarw.MnO.sub.2 +2MnO.sub.4 +4OH (alkaline) EQU MnO.sub.4.sup.- +2H.sub.2 O+3e.sup.- .fwdarw.MnO.sub.2 +2H2O (neutral)
Because the permanganate will not ionize to release the active permanganate ion unless water is present, the substrate must be hydrophilic and the reaction must take place in normal ambient humidity. The '399 patent teaches that the amount of water necessary to cause the oxidation reaction is supplied by a normal ambient humidity. There is no teaching or suggestion in the '399 patent to elevate the amount of free water in the pellet. Further, there is no teaching or suggestion in the '399 patent to elevate the concentration of permanganate in the pellet above that obtained with the 5% aqueous solution of permanganate.
The potassium permanganate impregnated alumina pellets of the '399 patent are limited in that they have a limited capacity for removing undesired contaminants from gas streams. In one specific example in the '399 patent, the adsorbent was impregnated with a 5% aqueous solution of the permanganate and subsequently dried. No particular concentration range of potassium permanganate or water is disclosed for the pellets of the '399 patent. As the '399 patent does not teach or suggest elevated concentrations of permanganate or free water in its pellets, the potassium permanganate impregnated alumina of the '399 patent appears to have the same limitations as the potassium permanganate impregnated alumina of the '750 patent as discussed above.
Yet another example of a solid filtration media for removing undesirable compounds from a gas stream is disclosed in U.S. Pat. No. 3,226,332. The '332 patent teaches a method of producing granular activated alumina uniformly impregnated with a solid oxidizing agent, preferably potassium permanganate, for use in treating fluid streams. This method includes the spray addition of the impregnate, wherein the impregnate solution is sprayed onto the dry combination being tumbled in a mixer thereby forming pellets which are later dried to remove a substantial portion of the remaining water. The preferred impregnated concentration of potassium permanganate is 2.5%, by dry weight. The '332 patent discloses a range of 0.5 to 10% of potassium permanganate, from about 2% to about 4% of potassium permanganate being preferred. The '332 patent also discloses that the pellets are dried to remove at least a substantial portion of the uncombined (free) water.
Although the permanganate impregnated alumina of the '332 patent may contain approximately 0.5 to 10% potassium permanganate, the preferred range of potassium permanganate remains at from about 2% to about 4%. Accordingly, if the concentration of the potassium permanganate in the solid substrate is above 4%, it does not appear that the patentee expects any significant improvement over its capacity at a concentration of 4%. Also, the '332 patent does not teach elevating the amount of water remaining in the substrate. Further, there is no teaching in the '332 patent of how to avoid clogging of the pores of the substrate as expected from the high concentration of potassium permanganate. Accordingly, the potassium impregnated alumina as taught in the '332 patent would be expected by one skilled in the art to be limited in its capacity for the removal of undesirable compounds from fluid streams, and therefor has the same shortcomings as the potassium impregnated alumina of the '750 patent as discussed above.
As seen above, it is believed in the prior art that the adsorptivity of permanganate impregnated alumina is maximized when the concentration of the potassium permanganate impregnated in the alumina is approximately 4-5%. It is further taught that at concentrations above 5%, the potassium permanganate crystallizes and plugs the pores in the media, and therefore does not increase and may even decrease the adsorptivity of the media.
For example, in a study entitled "Proposed Program For Stage I of Research at Colorado School of Mines Research Foundation" of Golden Co., for Marbon Chemical of Des Plaines, Ill., it was found that the acceptable range of MnO.sub.4.sup.- in alumina pellets was between 2.5 to 3.0%, calculated as MnO.sub.4.sup.- ion weight percent of the oven-dry alumina. The 3.0% cut-off was based on prior experience where attempts to store greater amounts of MnO.sub.4.sup.- in the pellets resulted in pore-plugging by small crystals of KMnO.sub.4.
Also, in a conference report between Borg Warner Corporation Research Center of Des Plaines, Ill., and Kaiser Aluminum and Chemical Corporation of Baton Rouge, La., dated Jan. 11, 1960, regarding a cooperative R&D program on substrate improvement, it was concluded that a permanganate ("MnO.sub.4.sup.- ") concentration of 2.0% (dry basis) appeared to be optimum. It was also concluded that for production purposes it would be better to aim for 1.75 to 2.00% since it was thought that the tendency toward pore-plugging increases very rapidly above 2%.
Also, as seen above, it is believed that the concentration of water should be minimized in solid filtration media. In fact, the industry continues to minimize the water content of such media. The resistance towards increasing the concentration of water in alumina filtration media results from the belief that the activity of the media is directly related to its surface area and pore size. Significantly increasing the water content would therefore be expected to reduce the surface area available for adsorption of contaminants, and therefore decrease the media's efficacy.
Although there are a variety of permanganate impregnated substrate known in the art for removing undesirable contaminants from fluid streams, as demonstrated above, these known impregnated substrates all have a limited capacity for the removal of undesirable compounds from gas streams, and therefore have limitations and drawbacks in their use, and do not meet the needs of various industries.
Therefore, what is needed is a high efficiency, high capacity, low flammability permanganate impregnated substrate for the removal of undesirable compounds from gas streams. Further, this impregnated substrate needs to be long lasting, requiring fewer replacements and thereby minimizing replacement and maintenance costs. Also needed is a high capacity impregnated substrate which may be used in small filter beds, and therefore may allow the treatment of fluid streams where there are significant space limitations.